This invention pertains to laser radiation sensor devices, and more particularly to a method and apparatus for detecting monochromatic (laser) radiation against a background of broad-band (ambient) radiation.
Advances in laser technology have produced lasers (notably dye lasers) whose radiation may be varied continuously over a broad portion of the visible and near IR spectrum. Since laser radiation is well adapted to such military applications as ranging and tracking, missile guidance, covert illumination, etc., the advent of the tuneable laser presents a problem to those who wish to detect laser radiation for threat-warning and countermeasure purposes.
To understand the requirements for a good Laser Detection System (LDS), the characteristics of the laser radiation likely to be encountered should be known. Laser radiation has the following characteristics:
1. The light has a very narrow bandwidth and is essentially monochromatic.
2. It is coherent.
3. It is usually fairly well collimated, i.e., narrow beam.
4. It may be in any state and degree of polarization, or it may be unpolarized.
5. Laser radiation used for ranging and tracking purposes will probably be pulsed. The pulses may be extremely short (on the order of nanoseconds) and the pulse repetition rate may be low (on the order of 10 pulses per second).
6. Although the laser source is likely to emit relatively high peak power, the received energy may be considerably less than from broad-band sources or ambient background.
Polarization effects of laser radiation have been . used to modulate the response of a detector receiving the radiation so that the output of the detector exhibits characteristics which are a function of the wavelength of the radiation received. However, the polarization optics are expensive and limit the detector's field-of-view.